Microstrip to microstrip transition

ABSTRACT

A coupling arrangement for microstrip systems, each system characterized by a dielectric with a ground plane contacting one side thereof and a conductive strip fastened to the other side. A small coupling hole in the ground plane immediately underlies the strip conductor at a current maximum for signal along the line. Two such microstrip systems are placed in back-to-back relation with their ground planes in physical contact and their coupling holes in registry to transfer signal being propagated in a transverse electromagnetic mode via one of the strip conductors to the other of the strip conductors and ultimately to a desired point or points of utilization. In a specific application of the invention, the coupled microstrip systems are used to transfer signal energy from a feed network to a phased array of antenna elements.

[ Nov. 6, 1973 1 MICROSTRIP TO MICROSTRIP TRANSITION [75] Inventor:Harry R. Phelan, lndialantic, Fla.

[73] Assignee: Harris-Intertype Corporation,

Cleveland, Ohio [22] Filed: May 25, 1971 [21] Appl. No.: 146,736

[52] US. Cl 333/84 M, 333/24 R [51] Int. Cl. 1101p 5/02, HOlp 3/08 [58]Field of Search 333/84 M, 84 R, 10', 333/73 W, 24 R, 6, 9; 317/101 CM,101 CE; 340/174 GP [56] References Cited UNITED STATES PATENTS 3,150,3369/1964 Gonda 333/84 M 2,654,842 10/1953 Engelmann.. 333/84 M 2,760,1698/1956 Engelmann 333/84 M 2,794,185 5/1957 Sichak 333/84 M 2,962,71611/1960 Engelmann.. 333/84 M 2,976,499 3/1961 Sferrazza 333/84 M3,012,210 12/1961 Nigg 333/84 M X 3,176,275 3/1965 Gribble et al....333/24 X 3,368,169 2/1968 Carter et a1. 333/73 S 3,587,110 6/1971Woodward 333/9 X FOREIGN PATENTS OR APPLICATIONS 828,241 2/1960 GreatBritain 333/10 159,017 2/1953 Australia 333/9 1,191,414 10/1959 France333/10 OTHER PUBLICATIONS Cohn, S. B. Slot Line on a DielectricSubstrate, M'IT-17 l01969, pp. 768-778.

Dukes, V. M. C. Broad-Band Slot-Coupled Micr0- strip DirectionalCouplers, IEE Vol. 105B, 1958, pp. 147-154.

Cohn, S, B. Microwave Coupling by Large Apertures Proc. IRE 6-1952, pp.696-698.

Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Wm. H. PunterAttarneyDonald R. Greene [57] ABSTRACT A coupling arrangement formicrostrip systems, each system characterized by a dielectric with aground plane contacting one side thereof and a conductive strip fastenedto the other side. A small coupling hole in the ground plane immediatelyunderlies the strip conductor at a current maximum for signal along theline. Two such microstrip systems are placed in back- 3 Claims, 7Drawing Figures PATENTEU NOV 6 i975 SHEET 1 CF 3 INVENTOR H. R. PHELAN 1MICROSTRIP TO MICROSTRIP TRANSITION BACKGROUND OF THE INVENTION 1. Fieldof the Invention The present invention relates generally to transmissionlines, and more particularly to microstrip circuits and transmissionlines and to techniques of coupling such lines.

2. Discussion of Prior Art As is well known, a strip transmission lineemploys conductors in the form of fiat strips or plates. In the typicalmicrostrip transmission line the conductive strip is separated from aground plane by a dielectric layer, and in general the circuit isfabricated in the form. ofa dielectric sheet clad on each sidewithcopper foil of predetermined width. The foil on one side constitutes theground plane, and the foil on the other side is the signal transmissionconductor strip. Microstrip lines find wideuse in signal transmission inthe transverse electromagnetic (TEM) mode at microwave frequencies. Acommon application of such lines is in the field of antenna systems, inwhich the microstrip line may be employed as part of a feed for a phasedarray, for example.

In the past, in order to couple a pair of microstrip circuits it hasbeen necessary to utilize radio frequency (RF) connectors.Implementation of a quick disconnect connector, as would be desirable,for example, for removal and replacement of faulty modules in a phasedarray antenna system is expensive and unreliable. Moreover, coaxial andpin connectors add considerable expense to the cost of the system inwhich theyare used, particularly where a large number of such connectorsare required, as in the typical phased array application.

SUMMARY OF THE INVENTION Briefly, according to the present invention,the ground plane of the microstrip circuit is provided with a narrowslot extending generally transverse to the direction of propagation ofsignal along the overlying conductive strip, and beyond the edge of thestrip generally symmetrically to either side thereof. In the case of anopen circuit line transition, the center of the slot is disposedapproximately a quarter wavelength (X/4) back from the terminus of theconductive strip constituting the end of the transmission line.Alternatively, as a transition for a short circuit line, the microstripmay be short circuited to the edge of a metallic carrier for themicrostrip circuit, immediately after crossing the slot (i.e., at theopposite side of the slot).

Two such microstrip circuits or transmission lines are readily coupledwithout need for the usual RF connectors by placing the circuits inback-to-back relation with the ground plane of each in contact with theground plane of the other and with the respective coupling slots inregistry. The end of the conductive strip of each transmission line mayextend to the same or to the opposite side of the registered slotsrelative to the end of the conductive strip of the other transmissionline. The conductive strip may undergo a turn, relative to itsorientation across the slot, after it has traversed a distance from theslot sufficient to preclude disruption of the field distribution at thetransition.

Accordingly, it is a principal object of the present invention toprovide a coupling arrangement, or transition, for microstriptransmission lines, by which good signal coupling is achieved betweenthe microstrip lines without need for conventional connectors.

It is another object of the invention to provide a microstrip tomicrostriptransition which achieves relatively localized couplingbetween lines at a point along the lines displaying a current maximum inthe signal wave, and yet with virtually negligible discontinuity in theground plane of each line.

BRIEF DESCRIPTION OF THE DRAWING In describing certain preferredembodiments of the invention, reference will be made to the accompanyingFIGURES of drawing, in which:

FIG. 1 is a fragmentary perspective view of an em bodiment of atransition for an open circuit line;

FIG. 2 is a section view taken along the line 22 of FIG. 1;

FIG. 3 is a detailed fragmentary section view of microstrip transmissionline illustrating the orientation of electric field lines for a signaltransmitted along the line in the transverse electromagnetic mode;

FIG. 4 is a section view taken along line 44 looking in the direction ofthe arrows in FIG. 5 of a microstrip to microstrip transition in whichthe circuits are maintained in place by a carrier medium;

FIG. 5 is a plan view taken along line 5-5 looking in the direction ofthe arrows in FIG. 4 of the arrangement of FIG. 4, showing severalembodiments of the transition according to the present invention;

FIG. 6 is a partial section view of a phased array antenna and feednetwork using microstrip to microstrip transitions;

FIG. 7 is a plan view of the configuration of FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1 and 2,each of a pair of microstrip lines l0, 11, which are shown as coupled inaccordance with the teachings of the present invention, comprises adielectric sheet or substrate 12 clad or bonded on each side thereofwith a layer of suitable signal conductive metal such as a copper foil.The substrate 12 may be aceramic, such as alumina (aluminum oxide). Themetal layer on one side of substrate 12 constitutes aground plane 13,while the metal layer on the other side of the substrate is a relativelynarrow strip 15, following the desired path of the transmission line orsigrial-carrying circuit.

According to the invention, a narrow slot 14 is provided in each groundplane 13 at a position immediately underlying the conductive strip 15and at an angle thereto, where a transition from one microstrip line toanother is desired. Although in FIGS. 1 and 2 the slot 14 is illustratedas being oriented at a right angle to the longitudinal path of the strip15, that orientation is not essential to the invention and it is to beunderstood that the slot may be canted or folded relative to thestrip.It is desired that a current maximum for signal propagated along theline exist in the vicinity of the slot in order for the slot to act as acoupling hole for the line. Such a current maximum occurs over thecoupling hole, as shown in FIGS. 1 and 2, along an open circuit linewhich extends a quarter wavelength ()t/4 or an odd multiple thereof atapproximately the center of the range of frequencies to be accommodatedby the microstrip line) beyond the slot. The longer dimension of theslot 14 is preferably about M2, but the narrower dimension is notcritical and may typically be about A; the size of the longer dimension,i.e., approximately M16.

Of importance to the present invention is the fact that signal ispropagated along the microstrip transmission line in a transverseelectromagnetic (TEM) mode, in which the TEM E-field configuration isillustrated by the dotted lines and arrows 17 in FIG. 3. When twomicrostrip transmission lines constructed as indicated above are placedin back-to-back configuration, as in FIGS. 1 and 2, with the groundplane 13 of each microstrip circuit in contact with the ground plane ofthe other and with the slots 14 in the respective ground planes inregistry, the strip conductors 15 of the two microstrip circuits areelectromagnetically coupled through the current maximum existing overthe coupling slot. Thus, a localized transition occurs at the otherwisenegligible discontinuity (i.e., the coupling slot) in the ground plane.In each instance, the slot may be vacant (i.e., open) or it may befilled with a dielectric insert. While the coupled lines are shown inFIGS. 1 and 2 with the ends of the strip conductors 15 extending inopposite directions beyond slot 14, it is to be understood that such anarrangement is not essential to the invention and that the strips mayterminate at the same side of the slot according to the desired path ofsignal into and away from the transition.

With reference to FIGS. 4 and 5, two such back-toback microstripcircuits 10, 11 are conveniently supported by sandwiching/or otherwiseelectrically connecting the ground planes of the two circuits betweenthe walls or blocks 19, 20 of a metal carrier. As is more clearly shownin FIG. 5, the metal carrier in this particular example has arectangular cross-section with an open center, and the microstripcircuits are held captive in the carrier by a plurality of hold-downfasteners 22. The arrangement shown in FIGS. 4 and constitutes a modulewhich is readily disassembled to permit rapid replacement of faultymicrostrip circuit portions.

Referring specifically to FIG. 5, several embodiments of transitions inaccordance with the present invention are shown by way of example toillustrate the versatility of the invention. The transition designated Ais substantially identical to that shown and described with reference toFIGS. 1 and 2. The transition denoted B illustrates the variation of thepath of strip conductor 15 which is possible, if desired, at either sideof coupling hole, but it is to be understood that a bend at any angleaway from the slot may be utilized, rather than a 90 angle, if desired.However, it is essential that the strip conductor not undergo a changein orientation within two strip widths from the side of the couplingslot at which the turn or bend is desired. This is because a smallerdistance between the point of the bend and the side of the slot maydisrupt the field distribution which is necessary at the point oftransition, thereby producing an impedance mismatch. Typically, thewidth of the strip is approximately 0.050 inch, but it is to beunderstood that other widths may be encountered in practice, and thatthe invention is not to be considered as limited to use of a strip ofspecific width.

A different embodiment of transition according to the invention isillustrated by the configuration designated C in FIG. 5. Here, therequired current maximum over coupling hole 14 occurs as a consequenceof the direct short circuit of the line to a point of referencepotential, i.e., ground, at the edge of the microstrip circuitsubstrate. In particular, the strip conductor 15, or

an extension electrically connected thereto, may project across couplinghole 14 to contact the conductive wall 20 of the circuit carrier.Reliable contact between the strip conductor and the metallic wall isassured by use of a screw fastener 24, as shown, which is threadedtightly down upon the projection strip conductor. Of course, other shortcircuiting techniques will readily suggest themselves to those skilledin the art, and may be employed as alternatives in the short circuitline configuration which has been described. The advantage of the shortcircuited line is its capacity for operation over much wider bandwidthsthan the quarter wavelength open circuit stub configurations A and B. Intheory, the short circuit configuration is frequency independent.However, the open circuit transition embodiment has the advantage ofsimplicity of implementation anywhere within the boundaries of themicrostrip circuit substrate, since it does not require a directmechanical shortcircuit to a point of ground potential.

Microstrip transition embodiment D (FIG. 5) is a version of the shortcircuit line similar to that designated B for the open circuittechnique, in that the strip conductor undergoes a bend at a pointexceeding two strip widths from the side of the coupling slot.

One example of an application of the microstrip-tomicrostrip transitiondescribed above is as a modular feed network for a phased array antenna.Such an arrangement, shown in FIGS. 6 and 7, eliminates the need for RFconnectors between the array modules and the feed network, as would berequired with conventional techniques. The feed network 30 comprises amicrostrip circuit 31 including a substrate 32 having a feed networkstrip 33 bonded thereto and which is itself attached to the wall 34 of ametallic enclosure 35 serving as the ground plane for the circuit. As ismore clearly shown in FIG. 7 the path of microstrip feed network line 33(dotted line) is defined according to the configuration of the phasedarray of radiating elements.

The array modules 40 comprise separate RF microstrip circuits of thetype which has generally been described with reference to FIGS. 4 and 5,and which comprise a dielectric substrate 41 fastened to a wall 42 of ametallic carrier serving as a ground plane. A strip conductor 44fastened to the opposite side of the substrate is directly connected toa radiating element 48 constituting one of the elements of the phasedarray. The ground plane of each array module 40 is provided with acoupling hole 45 which, when the module is fastened in appropriateposition to the feed network enclosure 35, registers with an associatedcoupling hole 37 in the ground plane formed by the feed networkenclosure. A set of two or more quick disconnect fasteners 50 isemployed with each module to permit the rapid and convenient removal andreplacement of any faulty module of the array, while requiring but asingle input-connector S1 for the entire feed network.

While the use of readily separable ground planes has been described, itshould be understood that the present invention is also applicable tothe coupling of microstrip circuits in which the coupled circuits sharea common ground plane. While such an arrangement does not have a quickdisconnect capability, it does possess the other advantages such asoverall simplicity of the transition and the absence of any requirementof a hole extending through the microstrip circuit substrate.

Accordingly, while certain preferred embodiments of the presentinvention have been disclosed herein, those skilled in the art to whichthe invention pertains will recognize that variations of the specificdetails of construction which have been illustrated and described may beresorted to without departing from the spirit and scope of theinvention, as defined by the appended claims.

I claim:

1. A direct coupling configuration for coupling the sole output of onemicrostrip line to the sole input of another microstrip line comprising:

first and second microstrip lines, each of said lines having only oneinput port for power and only one output port for power and eachincluding a strip conductor having a longitudinal centerline andseparated from a ground plane by a dielectric layer, for propagation ofsignal within a predetermined frequency range in a TEM mode via saidstrip conductor, and

a coupling hole in said ground plane at a point immediately oppositesaid strip conductor;

said first and second microstrip lines being disposed in back-to-backrelation with the ground plane of each in direct physical and electricalcontact with the other and the coupling holes in registry, and with saidcenterlines substantially parallel to each other at the place nearestsaid hole, to provide direct electromagnetic coupling between the twostrip conductors,

first means for terminating a first one of said microstrip lines in alow-loss impedance different from its characteristic impedance, withsaid coupling hole intennediate said input port of said first line andsaid means for terminating, to produce a standing wave thereon uponexcitation and to block any significant power transfer therebeyond, and

second means for terminating the second one of said microstrip lines ina low-loss impedance different from its characteristic impedance, withsaid coupling hole intermediate said output port of said second line andsaid second means for terminating, to produce a standing wave thereonupon excitation and to block any significant power transfer therebeyond,whereby said coupling hole is the sole power output port for said firstmicrostrip line and is the sole power input port for said secondmicrostrip line,

said coupling hole being a slot extending transverse to said stripconductors approximately one-half wavelength long in said transversedirection at the wavelength of signal to be accommodated by saidcircuits and being of a width substantially less than said length andsaid coupling hole of each microstrip circuit being located at a pointof current maximum for said standing wave on said strip conductor sothat essentially all of the input power applied to said first microstripcircuit is directly transferred to said second microstrip circuitthrough said coupling holes with no intermediate transmission medium.

2. The coupling configuration of claim 1, wherein at least one of thestrip conductors terminates approximately one-quarter wavelength beyondone side of the coupling hole, at the wavelength of signal to beaccommodated by said microstrip lines.

3. The coupling configuration of claim 1, wherein at least one of thestrip conductors terminates at one side of the coupling hole in a shortcircuit.

1. A direct coupling configuration for coupling the sole output of onemicrostrip line to the sole input of another microstrip line comprising:first and second microstrip lines, each of said lines having only oneinput port for power and only one output port for power and eachincluding a strip conductor having a longitudinal centerline andseparated from a ground plane by a dielectric layer, for propagation ofsignal within a predetermined frequency range in a TEM mode via saidstrip conductor, and a coupling hole in said ground plane at a pointimmediately opposite said strip conductor; said first and secondmicrostrip lines being disposed in backto-back relation with the groundplane of each in direct physical and electrical contact with the otherand the coupling holes in registry, and with said centerlinessubstantially parallel to each other at the place nearest said hole, toprovide direct electromagnetic coupling between the two stripconductors, first means for terminating a first one of said microstriplines in a low-loss impedance different from its characteristicimpedance, with said coupling hole intermediate said input port of saidfirst line and said means for terminating, to produce a standing wavethereon upon excitation and to block any significant power transfertherebeyond, and second means for terminating the second one of saidmicrostrip lines in a low-loss impedance different from itscharacteristic impedance, with said coupling hole intermediate saidoutput port of said second line and said second means for terminating,to produce a standing wave thereon upon excitation and to block anysignificant power transfer therebeyond, whereby said coupling hole isthe sole power output port for said first microstrip line and is thesole power input port for said second microstrip line, said couplinghole being a slot extending transverse to said strip conductorsapproximately one-half wavelength long in said transverse direction atthe wavelength of signal to be accommodated by said circuits and beingof a width substantially less than said length and said coupling hole ofeach microstrip circuit being located at a point of current maximum forsaid standing wave on said strip conductor so that essentially all ofthe input power applied to said first microstrip circuit is directlytransferred to said second microstrip circuit through said couplingholes with no intermediate transmission medium.
 2. The couplingconfiguration of claim 1, wherein at least one of the strip conductorsterminates approximately one-quarter wavelength beyond one side of thecoupling hole, at the wavelength of signal to be accommodated by saidmicrostrip lines.
 3. The coupling configuration of claim 1, wherein atleast one of the strip conductors terminates at one side of the couplinghole in a short circuit.